Neurotransmitters were localized to specific populations of axonal endings and neuronal perikarya in layer I and II of the medullary dorsal horn. Monoaminergic endings were characterized using light and electron microscopic autoradiography following uptake of (3H) serotonin or (3H) norepinephrine. The aminergic endings synapsed primarily on dendrites and occasionally on neuronal perikarya. These observations suggest that monoamines modulate the response of second order projection neurons to nociceptive primary inputs either directly through synapses on layer I projection neurons or indirectly through synapses on the interneurons in layer II. Experiments using anterograde transport of (3H) amino acids demonstrated that three types of endings originate from the medical brainstem. Each of these resembled endings labeled in the (3H) 5HT uptake experiments. Enkephalin and substance P were localized to axonal endings and neuronal perikarya in the dorsal horn using immunocytochemistry. Enkephalin neurons were found in layers I and II while substance P neurons were in layer II. A differential laminar distribution of endings was also observed.